A three year program of research is proposed to achieve greater insights into the nature of the responses of long bones to dynamic, intermittent compressive forces. The Hueter-Volkmann law of epiphyseal pressures describes an inverse relationship between static compressive forces parallel to the axis of epiphyseal growth and the rate of growth of that cartilage. This has been utilized in orthopaedic surgery in which epiphyseal stapling has afforded a means to slow longitudinal bone growth temporarily. In a previous study using experimental bipedalism, I showed that dynamic intermittent compressive forces stimulate the growth of epiphyseal cartilage. Researchers in this area have employed pins, springs, or hydraulic devices to produce compression on limb bones, usually on the tibia. The problem inherent in the employment of such devices is that the intermittent compressive forces are not produced when and how they naturally occur; what is produced is intermittent trauma. I propose to load the animals by using constant centrifugation. The cages will be mounted at various distances from the center of rotation such that each experimental group will experience a specific G effect for the duration of the experiment, beginning at 1.1 G and increasing by 0.1 G to 2.0 G. In this way, the limb bones of the animals of a specific group, e.g., those experiencing 1.5 G, will each experience the effects of an increase in body weight of exactly 50%; and such that the mechanism by which the limbs are loaded during locomotion would not differ from that in the control animals, only the magnitude would. Thirty-day-old male Sprague-Dawley rats will be subjected to constant centrifugation for sixty days; a 90-day-old rat is mature. There will be 11 groups of rats; one control (1.0 G), and one experimental group for each increase of 0.1 G, for a total of ten experimental groups. At sacrifice, each animal will be weighted and the limb bones removed, fixed, measured, weighed, and prepared for histological investigation to ascertain the effects of increased intermittent compressive forces on bone and bone growth. The second task in this research project is to see if the amount of loading expressed as a percentage of body weight which shows to be the most conducive to bone growth in normal rats, is the same percentage in clinical situations known to inhibit bone growth, i.e., hypophysectomized and rachitic rats.